Interface device between an ATM equipment and a transmission channel having a synchronous wireless link

ABSTRACT

The interfacing device handles asynchronous/synchronous adaptation and rate control between the ATM flows and the flows transmitted on the wireless interface. It transposes the ATM cells (53 octets) into packets which can be transmitted directly on the wireless interface (40 octets in the case of DECT). Similarly, at the receiving end, it reconstructs the original ATM cell flow on the basis of the packets received. In order to guarantee a certain transmission quality at the level of the wireless link, the device uses a data acknowledgement and retransmission mechanism adapted to the characteristics of the synchronous wireless channel. In order to fulfil the quality of service requirements of the various applications in terms of error rates and transmission delays, the device allows the data protection mechanism to be set for each ATM connection depending on the characteristics of the service.

BACKGROUND OF THE INVENTION

The present invention relates to Asynchronous Transfer Mode (ATM)transmissions. It specifically relates to providing such transmissionson synchronous wireless links.

ATM is a communication mode which enables numerous flows having variousrate and quality of service (delay and error rates) characteristics tobe multiplexed. For example, video flows are the most demanding streamssince they have a high speed and require a relatively low error rate (inthe order of 10⁻⁶) with very strict delay requirements.

The basic data units transmitted in ATM are called cells and consist ofpackets made up of 53 octets, i.e. 48 octets of payload preceded by a5-octet header.

Carrying a flow of ATM cells across a high-rate synchronous wirelessinterface gives rise to the following difficulties:

the wireless interface of the synchronous type must be capable ofsupporting an asynchronous data flow (ATM),

the size of the packets carried per timeslot on the radio interface doesnot generally correspond to that of the ATM cells,

the error rates which can be expected on such a wireless interface aremuch higher than what is desired for multimedia applications,

the delay incurred by the processing of the communication channel needsto be as short as possible in order to meet the strict requirements ofcertain applications in terms of transmission delay.

One of the types of wireless links to which the invention may be appliedis that of DECT links (Digital Enhanced Cordless Telecommunications).

The DECT radio interface was originally devised as a wireless access toa fixed network supplying each user with a rate of 32 kbit/s without anyprotection, essentially intended for transmitting speech (the so-callednon-protected mode). A protected mode also exists, which is intended forcarrying data and which involves applying a block code for correctingerrors, which has the disadvantage of requiring a complex decodingalgorithm (decoding for BCH codes). By using a multi-slot allocationtechnique, the standard nevertheless permits symmetrical or asymmetricalhigh-rate wireless connections, each transmission direction of which ismade up of a whole number of channels of 32 kbit/s without protection.Because of the simple channel coding, each wireless link offers an errorrate of only approximately 10⁻³.

As a result, using such links as a reliable means of carrying ATM cellsfor a variety of applications seems to be somewhat problematic.

A main object of the invention is to overcome this difficulty.

SUMMARY OF THE INVENTION

Accordingly, the invention proposes a interface device between an ATMequipment and a transmission channel having a synchronous wireless link.The interface device is arranged to process:

ATM cells;

segmented data units, each containing either data extracted from atleast one ATM cell and alignment data locating the starts of ATM cellsin said extracted data or stuffing data if no ATM cells are to betransmitted;

numbered data units, each containing a segmented data unit and numberingdata, at least some of the numbered data units further containingacknowledgement data; and

protected data units having the size of packets which can be transmittedon the wireless link and each having a data field for containing anumbered data unit and an integrity verification field for containing anintegrity verification code computed on the basis of said numbered dataunit.

The device comprises, for the transmission of an ATM cell flow on thetransmission channel:

a buffer memory receiving said ATM cell flow from the ATM equipment;

means for forming a first sequence of segmented data units, eachcontaining data read from the buffer memory;

means for forming a second sequence of numbered data units, eachcontaining a segmented data unit from the first sequence and numberingdata generated in the sequence order of the segmented data units of thefirst sequence;

means for forming a third sequence of protected data units, eachcontaining a numbered data unit from the second sequence and theintegrity verification code computed for said numbered data unit, thesequence order of the protected data units in the third sequence beingthat of the numbered data units which they contain in the secondsequence; and

means for transmitting the third sequence of protected data units on thetransmission channel.

The interface device further comprises, for receiving an ATM cell flowfrom the transmission channel:

means for receiving a fourth sequence of protected data units from thetransmission channel;

transmission error detection means for re-computing an integrityverification code on the basis of the contents of the data field of eachprotected data unit in the fourth sequence, comparing the re-computedcode with the contents of the integrity verification field of saidprotected data unit and indicating a transmission error for saidprotected data unit if the comparison reveals a difference;

means of forming a fifth sequence of numbered data units respectivelyextracted from the protected data units of the fourth sequence for whichno transmission error has been indicated;

means for forming a sixth sequence of segmented data units respectivelyextracted from the numbered data units of the fifth sequence; and

means for reconstructing an ATM cell flow output to the ATM equipment,on the basis of the data contained in the segmented data units of thesixth sequence, rearranged in accordance the numbering data contained inthe numbered data units of the fifth sequence.

The means for forming the second sequence of numbered data unitscomprise means for inserting acknowledgement data in the numbered dataunits of the second sequence, whereby the acknowledgement data isindicative of the protected data units of the fourth sequence for whicha transmission error has been indicated, and means for analysing theacknowledgement data contained in the numbered data units of the fifthsequence in order to insert in the second sequence of numbered dataunits repeats of numbered data units from the second sequence containedin protected data units of the third sequence for which the analysedacknowledgement data contained in the numbered data units of the fifthsequence indicate a transmission error.

Accordingly, the invention provides an adaptation layer between the ATMlayer and the wireless link medium (MAC layer—Medium Access Control). Itfulfils the following functions

1) Flow adaptation. The ATM flow transmission is asynchronous, whereasdata transmission on the wireless interface is synchronous. Certainapplications do not have specific rates (e.g. Internet) and cantherefore transmit data at a rate higher than that permitted on thewireless medium. The interface device takes over theasynchronous/synchronous adaptation and rate control between the ATMflows and the flows transmitted on the wireless interface.

2) Data adaptation. The device transposes the ATM cells (53 octets) intopackets that can be transmitted directly on the wireless interface (40octets in the case of DECT). Similarly, at the receiving end, itreconstructs the original ATM cell flow from the packets received.

3) Data protection. In order to guarantee a certain transmission qualityat the level of the wireless link, the device uses a dataacknowledgement and retransmission mechanism (ARQ, Automatic RepeatreQuest) adapted to the characteristics of the synchronous wirelesschannel. A more common way of resolving the problem inherent inprotecting data at the level of a wireless interface would be to protectthe data to be transmitted using an error correction code. In a frequentsituation where the channel is such that transmission errors occur bypackets, the data has to be interleaved beforehand so that once theinverse operation has been performed at the receiving end(de-interleaving), the errors are dispersed and the decoder can operatecorrectly. There is a conflict between this operation, which is betterthe longer the interleaving depth, and the transmission delayrequirement. Furthermore, error correction coding has the effect ofreducing the useful rate since it adds redundancy to the data to betransmitted.

4) Providing Quality of Service (QoS). In order to match up to thequality of service requirements of the applications in terms of errorrates and transmission delays, the device allows settings to beprogrammed for the data protection mechanism, for each ATM connection(service), depending on the characteristics of the service. The maximumpermissible number of retransmissions and the type of acknowledgement ofthe ARQ mechanism can be used to offer the required QoS in terms oferrors rates and delays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 are diagrams illustrating several possible applications forwhich the interface device proposed by the invention may be used.

FIG. 4 is a diagram illustrating the structure of data units processedby the device.

FIG. 5 is a block diagram of an embodiment of the device proposed by theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is described below as applied to ATM cell transmission viaa channel having a synchronous wireless link of the DECT type. Thedevice uses an adaptation layer, referred to hereafter as ADAL (ATM-DECTAdaptation Layer).

The ADAL interface may be used under various circumstances in thehigh-rate context. In the example illustrated in FIG. 1, interfacedevices 1 are positioned between DECT type stations 2 providinghigh-rate links and portions 3 of an ATM network, whereby a wirelesssupport may be offered to high-rate communications instead of a wireline in a fixed network.

In the application illustrated as an example in FIG. 2, interfacedevices 1, 4 according to the invention are arranged at the network endon the one hand and at the user end on the other, offering high-rateaccess to fixed users linked to the ATM access network via a localwireless loop. At the network end, the interface devices 1 are installedbetween the fixed ATM network 3 and the wireless stations 2 managed bythe operator. At the user end, the interface device 4 is located betweena DECT terminal 5 and the ATM terminal 6 of the user.

In the case of FIG. 3, the device 1 according to the invention provideshigh-rate wireless connections for users of a mobile network havingmultimedia mobile terminals 7. At the network end, the device 1 isinstalled in a manner similar to that illustrated in the example of FIG.2. In general, an ADAL layer will also be provided in the protocolssupported by the terminals 7.

In each of the examples illustrated in FIGS. 1 to 3, the channelestablished between two devices operating the ADAL layer is limited toone DECT wireless channel. In practice, the wireless channel may formonly a part of this transmission channel, being supplemented by othersupport channels (for example PCM, ATM, . . . ) capable of carrying theARQ packets described below.

The communication protocol between two remote ADAL layers enables an ATMflow to be conveyed on a connection established between these two ADALlayers and using a wireless link. Hereafter, this connection will bereferred to as an << ARQ connection>>. Packets transmitted at the rateof ARQ frames, referred to as << ARQ packets>>, circulate on this ARQconnection.

Since the wireless links used are of the DECT type, the ARQ packets areblocks of 40 octets generated between two ADAL layers. These blocks canbe transmitted directly on the wireless interface at the level of theB-fields of the DECT frames in question via the wireless connection. AnARQ packet therefore corresponds exactly to a DECT packet.

The ARQ packets are emitted at the rate of ARQ frames corresponding tothe transmission capacity of the DECT wireless connection used.Accordingly, each ARQ frame, made up of a fixed number of ARQ packets,is transmitted synchronously on a DECT frame, every 10 ms. If the DECTchannel allocated to the ARQ connection is made up of n elementarytimeslots per DECT frame for a rate of n×32 kbit/s, the ARQ frame willbe made up of n ARQ packets emitted in the timeslots in question duringthe DECT frame.

The ADAL layer used in an interface device according to the invention ismade up of three sub-layers:

a Protection And Detection sub-layer (PAD)

a Retransmission And Acknowledgement sub-layer (RAA); and

a Segmentation And Reassembly sub-layer (SAR).

The structure of the data exchanged between the different layers isillustrated by the diagram of FIG. 4, whilst FIG. 5 shows how theselayers are organised in one embodiment of the interface device accordingto the invention.

The MAC layer operated in the DECT interface 50 processes the ARQpackets forming the 40-octet DECT packets exchanged on the ARQconnection. The functions of this MAC layer includeestablishment/release of wireless connections, radio data transport andtransmission of the DECT frame synchronisation to the PAD layer.

The ARQ packet forms a protected data unit comprising a 38-octet datafield and a 2-octet integrity verification field.

The PAD sub-layer 40 controls the integrity of the ARQ packets as theyare transported on the ARQ connection. It sets up these packets with the38 octets of the numbered data unit PadSdu received from the RAAsub-layer supplemented by a cyclic redundancy checksum (CRC) inserted inthe integrity verification field. The PAD sub-layer also handles thetransmission and reception functions of the ARQ packets on a specifictransmission medium, optionally linking it to the DECT interface 50, andtransmits the 10 ms DECT synchronisation to the RAA sub-layer.

The elements of the interface device pertaining to the PAD sub-layer 40are schematically illustrated in FIG. 5. The multiplexer 41 forms theARQ packets by adding to the numbered data unit PadSdu received from theRAA sub-layer a CRC conventionally computed by a module 42 on the basisof the data contained in this unit of 38 octets.

At the receiving end, the demultiplexer 43 separates the numbered dataunit received and the CRC received. An error detection module 44re-computes the CRC on the basis of the data unit PadSdu received andcompares it with the CRC received in order to detect any reception errorwhich might have occurred in the binary data V sent to the RAAsub-layer.

In terms of protocol, three primitives are exchanged between the PAD andRAA sub-layers:

a primitive BR (Block Request), sent by the RAA sub-layer to the PADsub-layer to request transmission of a data block PadSdu of 38 octetssupplied as a parameter of this primitive;

a primitive BR (Block Indication) sent by the PAD sub-layer to the RAAsub-layer to indicate that it has received the data block PadSdu inquestion as well as the binary validity data V;

a primitive PAD_SYNCH.ind sent by the PAD sub-layer to the RAA sub-layerin order to synchronise this latter with the start of the ARQ frames.

The RAA sub-layer 30 manages the synchronous transmission of the ARQpackets, the part relating to the retransmission and acknowledgementmechanism and absorption of the delay spread generated byretransmission. The numbered data unit PadSdu managed by the RAAsub-layer is made up of 38 octets (FIG. 4), comprising:

optional acknowledgement data ACK, of a size variable between 0 and 6octets,

a packet numbering octet NUM used to retransmit the packets selectively,

a payload part RaaSdu which corresponds to the segmented data unitissued by the SAR sub-layer. The size of the payload part RaaSduintegrated in each data unit PadSdu will depend on the possibleacknowledgement to be inserted therein to ensure that the total lengthis always 38 octets.

The acknowledgement data ACK returned is computed ARQ frame by ARQframe. It indicates how each ARQ packet of a same ARQ frame wasreceived. In order to keep the size of the acknowledgements to aminimum, the ARQ packets are not acknowledged randomly but in a relativemanner having regard to the sequence number in the ARQ frame (thissequence number is distinct from the numbering data NUM incorporated inthe data units PadSdu). This means that the ARQ connections guaranteethe non-loss, non-insertion and correct sequencing of the ARQ packetsand that the protocol used at the transmitter level will retain inmemory the composition of the ARQ frames transmitted.

The protocol may operate in two acknowledgement modes: positive ornegative. In negative acknowledgement mode, it considers, by default,that the ARQ packets transmitted have been correctly received. In thismode, the transmission and the delay in the event that theacknowledgement data has not been received from the other party islimited but certain ARQ packets incorrectly received might end up notbeing transmitted. In positive acknowledgement mode, the situation isthe reverse: the packets are considered as having been incorrectlyreceived by default. This is more reliable but ARQ packets which werecorrectly received at the other end may sometimes be retransmitted.

The acknowledgement mode used may be selected when the ARQ connection isestablished. The negative mode is better suited to high rates andstringent delay requirements but implies a higher error rate than thepositive mode.

The acknowledgements are transmitted at the level of the wirelessinterface within the band, i.e. they are integrated directly in the flowof payload data circulating between the two peer ADAL interfaces. Thefact of carrying the acknowledgement data in the B-fields of the DECTframes ensures that they are carried in all the phases of thecommunication, including any handover.

The acknowledgement data is inserted at the beginning of the first AROpacket of each ARQ frame. Accordingly, the part available for thesegmented data units RaaSdu in the first packet of each frame is reducedby the size occupied by the acknowledgement data. This being the case,the RM sub-layer issues a request to the SAR sub-layer to sent it a dataunit RaaSdu whose size is 37 octets minus the size of theacknowledgement field. For the other ARQ packets in the frame, the RAAsub-layer issues a request to the SAR sub-layer for data units RaaSdu of37 octets. In order to limit the bandwidth occupied by acknowledgementdata, the size of this data varies between 1 and 6 octets and iscomputed depending on the rate reserved for the connection in theopposite direction. The minimum number of bits needed to acknowledge anARQ frame (in blocks of 8 bits to obtain a whole number of octets) isthe number of ARQ packets forming the ARQ frame received.

The acknowledgement data bits corresponding to the acknowledgement ofARQ packets are set to 1 if the ARQ packet was correctly received and to0 otherwise. The payload bits, corresponding to the acknowledgement of apacket whether correctly received or not, are aligned on the rightrelative to the octets reserved for the acknowledgement data.

The ARQ frames may have a single or double acknowledgement. The doubleacknowledgement consists in that the acknowledgement data inserted inthe first packet of each ARQ frame acknowledges two separate ARQ framesreceived in the opposite direction and in that each ARQ frame receivedis acknowledged by two ARQ frames transmitted in the opposite direction.

The double acknowledgement contained in each transmitted ARQ framerelates two ARQ frames received, consecutive or not. The gap betweenthese two ARQ frames received is a parameter which can be controlledwhen the ARQ connection is established depending on the applicationrequirements and the radio characteristics of the environment.

The single acknowledgement is not transmitted with more protection thanthe user data. Using a double acknowledgement therefore increasessecurity for the acknowledgements.

The double acknowledgement increases the retransmission time andbandwidth occupancy. The choice as to type of acknowledgement, single ordouble, and the corresponding parameters can therefore be used as ameans of adjusting the quality of service offered.

Another parameter linked to the quality of service is the maximum numberof retransmissions possible for a given packet. At the transmitter end,this number is used to limit the number of times a given ARQ packet isretransmitted. It also allows the receiver to determine the maximum timethat transmission of an ARQ packet can take. So as not to clog up theband for no reason, empty ARQ packets are not retransmitted.

In the RAA sub-layer 30 (FIG. 5), the multiplexer 31 assembles thenumbered data units, which are supplied to the PAD sub-layer 40 andstored temporarily in a memory 32. The module 33 which manages the ARQframes takes over the synchronisation functions in the RAA sub-layer. Itcontrols the multiplexer 31 so that the acknowledgement data ACK,computed by a module 34 on the basis of the validity binary data Vreceived from the PAD sub-layer PAD 40, is inserted at the start of thefirst packet of each ARQ frame. The module 33 also issues the numberingdata NUM on 8 bits which allows the transmitted data unit to be locatedrelative to the sequence as a whole.

The module 33 also receives from the multiplexer 35 the acknowledgementdata ACK received in the opposite direction. It analyses this data inorder to control the process of reading from the memory 32 each packetto be retransmitted, i.e. each packet for which an acknowledgement bitset to 0 was received, or for which the acknowledgement data wasincorrectly received (which the validity binary data V indicates for thefirst packet of the ARQ frames received) if acknowledgement is beingoperated in positive mode. Any packet read in the memory 32 is sent tothe multiplexer 31 rather than a packet picked up from the SARsub-layer.

In order to limit retransmission delays, the system is set up so that,in each frame, any packet retransmissions are dealt with first, startingwith those whose number NUM is the earliest, and the new packets whosepayload parts are requested from the SAR layer are not formed until allthe retransmissions have been completed.

In the receiving direction, the demultiplexer 35 separates the segmenteddata units RaaSdu, the numbering data NUM and, for the first packet ofeach ARQ frame, the acknowledgement data ACK supplied to the module 33.The control module 36 processes the numbering data NUM and the validitydata V relating to the packets received in order to control re-assemblyof the ATM flow. If a numbered packet was correctly received, a commandis issued to write its payload part into the ATM flow to be sent.Otherwise, the write procedure is inhibited until the packet has beenreceived incorrectly a number of times equal to the maximum number ofretransmissions stipulated when the connection was established.

In terms of protocol, the interface between the RAA and SAR layerscomprises three primitives:

a primitive DR (Data Request) sent by the RAA sub-layer to the SARsub-layer to request transmission of a segmented data unit RaaSdu of asize S depending on its position in the ARQ frame;

a primitive DG (Data Granted), sent by the SAR sub-layer to the RAAsub-layer in response, supplying a data unit to be transmitted; and

a primitive DI (Data Indication), sent by the RAA sub-layer to the SARsub-layer to indicate reception of a data unit RaaSdu, of a size Sdepending on its position in the ARQ frame received, and validity dataindicated by the bit V.

The SAR sub-layer 20 handles the segmentation and re-assembly of the ATMcell flow for despatch on the wireless link. It handles the dataadaptation part of the ARQ protocol: decomposition/recomposition of theATM flow and supply of the payload integrated in the ARQ packets. TheSAR sub-layer manages the segmented data units RaaSdu, the size of whichvaries from 31 to 37 octets for the reasons explained above.

These data units (FIG. 4) are made up of one octet corresponding toalignment data CI locating the data relative to the sequence of ATMcells, followed by a field of payload data of variable size (30 to 36octets) extracted from the transported ATM cell flow.

The SAR sub-layer 20 retains the ATM cell flow supplied by the ATMinterface 10 in a buffer memory and transmits it in blocks to the RAAsub-layer when requested to do so by the latter. A multiplexer 22 addsto the data extracted from the ATM cells and read from the buffer memory21 the alignment data CI issued by a segmentation module 23. This module23 receives the requests for data blocks of a size S from the RAAsub-layer and issues a command for this data to be extracted from thebuffer memory 21 accordingly. The alignment data CI is updated inaccordance with the position of the current data relative to theboundaries between successive ATM cells. By way of example, thealignment data octet CI may be broken down into a two-bit field TYP,indicating the type of block, and a 6-bit location field SOC coded asfollows:

TYP=00, SOC=000000: non-assigned block, i.e. the buffer memory 21 doesnot contain any data at the time of a request from the RAA sub-layer;

TYP=10: partially assigned block, i.e. containing no payload up to aposition, ranging between 1 and 35 octets, denoted by the field SOC;

TYP=11, SOC=111111: the data part of the RaaSdu block extends across asingle ATM cell which does not start in this RaaSdu block;

TYP=11, SOC=000000: the data part of the RaaSdu block commences right atthe start of an ATM cell; and

TYP=11, SOC≠111111 and 000000: the data part of the RaaSdu block spanstwo successive cells, the position of the beginning of the second cell,ranging between 2 and 36 octets, being indicated by the field SOC.

When the module 33 managing the ARQ frames reads a packet to beretransmitted in the memory 32 of the RAA sub-layer, it examines thealignment data octet CI contained in this packet to inhibit itsretransmission if it is a non-assigned block, in order to avoid anyuseless delays.

In the part pertaining to the SAR sub-layer 20, the interface devicealso has a demultiplexer 24 to which the segmented data units RaaSdureceived from the RAA layer 30 are applied, and which extracts thealignment data CI and the payload therefrom. A re-assembly module 26,controlled by the module 36 of the RAA layer, receives the extractedalignment data CI, the validity bit at V and the indication S as to thesize of the payload part of the current packet. Depending on thisinformation, the re-assembly module 26 issues a command to write thepayload to the appropriate locations of a memory 25 in which the ATMcells are reconstructed before being sent to the ATM interface 10. Themodule 26 may also indicate to the ATM interface which cells are likelyto contain errors on the basis of the validity bits V.

Two modes of carrying ATM cells may be used in the ADAL interface: anormal mode and an optimised mode.

In normal mode, the entire contents of the ATM cells are written to thebuffer memory 21 to be inserted in the segmented data units RaaSdu andin the ARQ data packets, except for the octet HEC carrying the ATMheader error detection code. Consequently, the 48 octets of the payloadpart of each cell are transported along with the first four octets ofthe header. Once transported, the 52-octet cells are reformatted by themodule 26 into real 53-octet ATM cells after inserting the HEC octet,which is initialised to 0 by default.

The optimised mode permits to cope with the relatively low ratesavailable on the DECT wireless interface. In this mode, the headers ofthe ATM cells are reduced to two octets, namely 4 bits for the VPI, 8bits for the VCI and 4 bits for the PT/CLP field (Payload Type/Cell LossPriority). Other bits are thus removed in addition to the HEC octet,namely certain virtual path and virtual channel identifier bits VPI/VCIand the GFC bits (Generic Flow Control). After they have beentransported across an ARQ connection, the reduced cells are reformattedinto complete cells by the module 26. The GFC fields, the HEC and theother non-transported parts of the ATM header are initialised to 0 bydefault during this operation. Using these reduced cells has the effectof limiting the number of bits which may be used to identify the virtualconnections and the loss of information contained in the GFC field. Thepresence of the PT/CLP fields in the header of the reduced cells allowsthe ATM cells to be transported in the ML5 format.

Normal mode allows a gain of one octet out of 53, which corresponds to again of 1.9% on the net bandwidth available for the user applications.The optimised mode increases this gain to 5.7%

What is claimed is:
 1. An interface device between an ATM equipment anda transmission channel having a synchronous wireless link, the interfacedevice being arrange to process: ATM cells; segmented data units, eachcontaining either data extracted from at least one ATM cell andalignment data locating starts of ATM cells in said extracted data orstuffing data if no ATM cells are to be transmitted; numbered dataunits, each containing a segmented data unit and numbering data, atleast some of the numbered data units further containing acknowledgementdata; and protected data units having the size of packets which can betransmitted on the wireless link and each having a data field forcontaining a numbered data unit and an integrity verification field forcontaining an integrity verification code computed on the basis of saidnumbered data unit; the interface device comprising, for thetransmission of an ATM cell flow on the transmission channel: a buffermemory receiving said ATM cell flow from the ATM equipment; means forforming a first sequence of segmented data units, each containing dataread from the buffer memory; means for forming a second sequence ofnumbered data units, each containing a segmented data unit from thefirst sequence and numbering data generated in the sequence order of thesegmented data units of the first sequence; means for forming a thirdsequence of protected data units, each containing a numbered data unitfrom the second sequence and the integrity verification code computedfor said numbered data unit, the sequence order of the protected dataunits in the third sequence being that of the numbered data units whichthey contain in the second sequence; and means for transmitting thethird sequence of protected data units on the transmission channel; theinterface device comprising, for receiving an ATM cell flow from thetransmission channel: means for receiving a fourth sequence of protecteddata units from the transmission channel; transmission error detectionmeans for re-computing an integrity verification code on the basis ofthe contents of the data field of each protected data units in thefourth sequence, comparing the re-computed code with the contents of theintegrity verification field of said protected data unit and indicatinga transmission error for said protected data unit if the comparisonreveals a difference; means for forming a fifth sequence of numbereddata units respectively extracted from the protected data units of thefourth sequence for which no transmission error has been indicated;means for forming a sixth sequence of segmented data units respectivelyextracted from the numbered data units of the fifth sequence; and meansfor reconstituting an ATM cell flow output to the ATM equipment, on thebasis of the data contained in the segmented data units of the sixthsequence, rearranged in accordance the numbering data contained in thenumbered data units of the fifth sequence, wherein the means for formingthe second sequence of numbered data units comprise means for insertingacknowledgement data in the numbered data units of the second sequence,whereby the acknowledgement data is indicative of the protected dataunits of the fourth sequence for which a transmission error has beenindicated, and means for analysing the acknowledgement data contained inthe numbered data units of the fifth sequence in order to insert in thesecond sequence of numbered data units repeats of numbered data unitsfrom the second sequence contained in protected data units of the thirdsequence for which the analysed acknowledgement data contained in thenumbered data units of the fifth sequence indicate a transmission error.2. A device as claimed in claim 1, wherein the numbered data units ofthe second sequence are grouped in frames, as are the protected dataunits of the third and fourth sequences, and wherein acknowledgementdata relating to a frame of the fourth sequence is inserted in the firstnumbered data unit of each frame of the second sequence.
 3. A device asclaimed in claim 2, wherein the means for analysing the acknowledgementdata are arranged to cause the numbered data units of a frame from thesecond sequence to be repeated if the error detection means indicates atransmission error for the first protected data unit of a frame of thefourth sequence containing acknowledgement data relating to said frameof the second sequence.
 4. A device as claims in claim 2, wherein themeans for inserting acknowledgement data are arranged to insert theacknowledgement data, indicative of the protected data units of a frameof the fourth sequence for which a transmission error has beenindicated, in the first numbered data units of the least two frames ofthe second sequence.
 5. A device as claimed in claim 1, wherein theacknowledgement data occupies a number of bits depends on the availablerate on the wireless link.
 6. A device as claimed in claim 1, wherein aheader error detection coding octet is removed from the data extractedfrom each ATM cell to be contained in a segmented data unit.
 7. A deviceas claimed in claim 6, wherein other bits of the header of each ATM cellare removed from the data extracted from each ATM cell to be containedin a segmented data unit, the other removed bits comprising virtual pathand virtual channel identifier bits and/or flow control bits.
 8. Adevice as claimed in claim 1, wherein the maximum number of times anumbered data unit from the second sequence can be repeated is aprogrammable parameter.